But New York University researchers found vitamins including E, C and beta carotene stop the liver breaking down an early form of bad cholesterol.

Writing in the Journal of Clinical Investigation, the researchers say their findings mean they cannot recommend that people use the vitamins.

Most research tends to suggest that supplementation with antioxidant vitamins, although not beneficial, does not lead to undue harm

Belinda Linden, British Heart Foundation

The vitamins are antioxidants, thought to be beneficial because they attack free radicals, produced when the body fights infection, which inflict damage on the body's tissues.

But studies carried out by the researchers at the university's school of medicine found that antioxidants actually hampered the body's fight against damaging cholesterol.

Normally, liver cells break down a key protein in harmful lipoproteins such as VLDL (very low density lipoprotein) which means they cannot be converted into a form of LDL that can enter the bloodstream.

However, in laboratory tests, the New York researchers found vitamin E, C and beta carotene prevented this process taking place in liver cells.

Dr Edward Fisher, director of the Lipid Treatment & Research Center at the NYU Medical Center, said: "Our study is the first to document this association between antioxidant vitamins and VLDL cholesterol.

"It does appear that antioxidant vitamins may be potentially harmful for the heart based on their ability to increase the secretion of VLDL in the liver cells and in the mice that we studied."

But he added: "More studies are needed to back up our findings. Until more data becomes available, we can't make any recommendations about whether people should not use these vitamins."

However he said there was evidence from other animal studies that antioxidants could have beneficial effects on other parts of the body, such as protecting the arteries from atherosclerosis and the pancreas and other organs from damage caused by diabetes.

Writing in the journal, Dr Ronald Krauss of Children's Hospital Oakland Research Institute in California, added that, although there was "considerable" evidence for the benefits of antioxidants, "the potential for unintended outcomes of oxidant therapy should serve as a warning against proceeding with such treatment in the absence of clinical-trial evidence of benefit and safety".

Belinda Linden, head of Medical Information at the British Heart Foundation (BHF), told BBC News Online: "Most research tends to suggest that supplementation with antioxidant vitamins, although not beneficial, does not lead to undue harm.

"Before any clear conclusions can be drawn from this study we would await the results of larger randomized controlled clinical trials."